title: Oligomerization of the extracellular domain of Boss enhances its binding to the Sevenless receptor and its antagonistic effect on R7 induction.
abstract: In the developing compound eye of Drosophila, neuronal differentiation of the R7 photoreceptor cell is induced by the interaction of the receptor tyrosine kinase Sevenless with its ligand Bride of sevenless (Boss), which is expressed on the neighboring R8 cell. Boss is an unusual ligand of a receptor tyrosine kinase: it is composed of a large extracellular domain, a transmembrane domain with seven membrane-spanning segments and a cytoplasmic tail. Expression of a monomeric, secreted form of the extracellular domain of Boss is not sufficient for Sevenless activation, and instead acts as a weak antagonist. Because oligomerization appears to be a critical step in the activation of receptor tyrosine kinases, we used oligomerized forms of the Boss extracellular domain to test their ability to bind to Sevenless in vivo and restore R7 induction in vivo. Oligomerization was achieved by fusion to the leucine zipper of the yeast transcription factor GCN4 or to the tetramerization helix of Lac repressor. Binding of these multivalent proteins to Sevenless could be detected in vitro by immunoprecipitation of cross-linked ligand/receptor complexes and in vivo by receptor-dependent ligand localization. However, neither R8-specific or ubiquitous expression of multivalent Exboss ligands rescued the boss phenotype. Instead, these ligands acted as competitive inhibitors for wild-type Boss protein and thereby suppressed R7 induction. Therefore the role of the transmembrane or cytoplasmic domains of Boss in the activation of the Sev receptor cannot be replaced by oligomerization.
authors.affiliation: Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallasp65235-9111, USA.
authors.forename: E A
authors.initials: EA
authors.lastname: Sevrioukov
authors.forename: J H
authors.initials: JH
authors.lastname: Walenta
authors.forename: A
authors.initials: A
authors.lastname: Sunio
authors.forename: M
authors.initials: M
authors.lastname: Phistry
authors.forename: H
authors.initials: H
authors.lastname: Krämer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified; D001667:Binding, Competitive; D002454:Cell Differentiation; D002460:Cell Line; D004268:DNA-Binding Proteins; D019281:Dimerization; D004330:Drosophila; D029721:Drosophila Proteins; D005110:Extracellular Space / Q000737:chemistry / Q000378:metabolism; D005123:Eye / Q000378:metabolism; D005136:Eye Proteins / Q000378:metabolism* / Q000502:physiology*; D005656:Fungal Proteins / Q000378:metabolism; D008024:Ligands; D008562:Membrane Glycoproteins / Q000378:metabolism* / Q000502:physiology*; D008969:Molecular Sequence Data; D017956:Photoreceptor Cells, Invertebrate / Q000166:cytology* / Q000254:growth & development / Q000378:metabolism; D011485:Protein Binding; D011494:Protein Kinases / Q000378:metabolism; D017433:Protein Structure, Secondary; D020794:Receptor Protein-Tyrosine Kinases; D018000:Receptors, Peptide; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins
chemical_list: D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D005136:Eye Proteins; D005656:Fungal Proteins; D008024:Ligands; D008562:Membrane Glycoproteins; D018000:Receptors, Peptide; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins; C072141:boss protein, Drosophila; D011494:Protein Kinases; D020794:Receptor Protein-Tyrosine Kinases; C056737:sev protein, Drosophila